Prior to this invention, alkali metal chlorates were typically produced in separate unipolar electrolyzer cells connected in electrical parallel, such as illustrated and described, for example, in U.S. Pat. No. 3,539,486, with the feed electrolyte solution of the cells being circulated and recirculated to and from a single reactor vessel. The chlorate concentration within the reactor vessel was permitted to build up and was maintained at a high concentration, and a small amont of electrolyte was continuously withdrawn to the precipitator crystallizer system, where by lowered temperature chlorate crystals were crystallized and separated from the alkali metal chloride solution which was returned to the electrolyzer cells. In these systems, the cell covers were provided with a gas duct system to vent the by-product gas, such as hydrogen contaminated with oxygen and chlorine, to the atmosphere or to a recovery and purification system. Other cells were sealed and operated with a high hydrogen content in the vent gas. Also, some prior systems made use of bipolar cells, introducing current into a terminal cell anode of a series of the bipolar cells, and the current leaving from a cathode of the terminal cell at the opposite end of the series For sodium chlorate production, usual current densities range from about 1,000 to 3,000 amperes per square meter, giving operating temperatures of about 60.degree. to 65.degree. C, but cathode densities may vary from about 500 to 2,000 A/m.sup.2 and anode current densities from about 500 to 4,000 A/m.sup.2. With smooth platinum anodes, a current efficiency of about 85% to 90% has been obtainable, and with platinized platinum, up to about 99% current efficiency.
In such systems, current efficiency improves at temperatures ranging downward to about 30.degree. C, with most cells operating within the range of 35.degree. to 45.degree. C, because of the greater wear on graphite anodes at the higher temperatures. Increases in current densities tends to increase voltage drop across a cell but to improve current efficiency and also to increase the rate of attack on graphite anodes per unit of time but to decrease attack on the graphite per unit of chlorate produced. In such systems, the current efficiency for sodium chlorate is about 80 to 85%, normally on a basis of a cell voltage of 3.6 with an energy expenditure of about 2.4 to 3 kwh/lb chlorate. It has also been customary to add chromate to passivate iron cathodes and other iron portions of the cell and reduce corrosion and to add HCl to maintain the electrolyte on the acid side.
Although current efficiency is improved at lower temperatures, the velocity of reaction of hypochlorous acid with sodium hypochlorite to form chlorate is known to increase particularly at temperatures about 70.degree. C. The chemical formation of chlorate takes place throughout the whole volume of the electrolyte and, therefore, it is recognized the current efficiency and chemical formation of chlorate are improved by greater current concentration per given area, and increasing electrolyte volume increases current efficiency.
In typical prior art systems, the space for electrolyte volume between anode and cathode surfaces (electrolyte gap) usually ranges between 0.25 to 0.5 in. (0.6 to 1.2 cm).
The chlorate industry is highly competitive and with increase in demand for sodium and potassium chlorate, it has become increasingly important to effect improvements in the apparatus and process of producing these chlorates, particularly sodium chlorate, to reduce the cost of production. However, there are complicating factors involved, such as higher current density normally resulting in higher cost of production, whereas at lower temperatures although current efficiency is increased, the rate of chemical formation of chlorate from hypochlorous acid and the hypochlorite ion decreases, i.e., the rate of chemical disproportionation decreases. In order to efficiently remove the chlorate by crystallization, it has been necessary either to operate the reactor and recirculated electrolyte solution at a high chlorate concentration and simultaneously the maximum level of sodium chloride, or to operate with a series of unipolar cells with the flowing electrolyte forming a cascade from cell to cell. Because of the necessarily high chlorate concentration in the first case, as well as the merely minor increase in chlorate concentration resulting from any one cell, high chlorate concentration has been a controlling factor with no apparent significant regard to the effect that this has on the efficiency of electrolytic decomposition of the sodium chloride. Also, although current efficiencies are greater at lower temperatures, the corresponding decrease in the rate of the disproportionation reaction requires a greater time of production of a given amount of chlorate with a corresponding increase in overhead costs per unit of time increase in direct proportion to additional time required for production of a given quantity of chlorate.